The gut microbiota has a significant role in human health and disease where dysbiosis of the intestinal ecosystem contributes to the development of certain illnesses varying from infectious antibiotic-associated diarrhea to colon cancer and obesity. Due to the fact that probiotics have been shown to positively influence gut health, the re- establishment of the balance by these bacteria for disease treatment and prevention have been used and studied in various disease areas. In this thesis I have tried to collect material for an overview of probiotics, prebiotics and synbiotics, but I also wanted to get a better understanding of how obesity could possibly be caused by this dysbiosis too.